The present invention relates to a fast radio frequency modulator and more exactly to a method and an arrangement for modulating a radio frequency signal by means of an arrangement utilizing a ferroelectric device.
In microwave applications for communication there is always a need for a modulation system which allows a fast information transfer. The modulated signal may basically be either phase/frequency modulated (PM/FM) or amplitude modulated (AM). In the conventional way this is solved by generating and modulating a carrier at a lower frequency and by means of some process transfer this modulated lower frequency to a radio wave at a much higher frequency. This transfer process for the actual modulation information may be obtained for instance by a multiplication of the modulated carrier and a microwave carrier in a mixer circuitry, whereby the desired frequency product is either the sum or the difference of the two carriers. However this conventional method calls for additional filtering to exclude the unwanted product frequency as well as the initial carrier frequencies.
Yet another conventional method would be to just frequency multiply the modulated carrier itself up to the desired frequency. The frequency multiplication is more often used in combination with phase modulation but still this method will be considered to have a number of disadvantages regarding for instance stability, bandwidth etc.
In an international application WO94/13028 is described different ferroelectric films such as SrTiO3, Pb(Sr,Ti)O3, SrxBa1-xTiO3, which provide a tunable dielectric constant by variation of a voltage applied across the ferroelectric film. Because of the intrinsically low losses at high frequencies, these materials can be implemented in a variety of microwave components. This document is hereby incorporated by reference.
When investigating the state of the art several documents are found discussing for instance phase-shifting. A microwave phase-shifter, which contains a transmission line section including a ferroelectric material on a dielectric backing is for instance described in a Russian document SU 1352562. This phase shifter further contains a second transmission line section electromagnetically coupled to the first section and having absorbent elements at the ends, or on the coupling part, of the second line section. However, it is claimed that various combinations of amplitude and phase modulation may be obtained, but the document neither indicates any detailed practically working modulator utilizing such a ferroelectric material, nor is any suitable material for such a microwave operation disclosed.
Another Russian document, SU 1193738, discloses a microwave phase shifting network which includes a dielectric substrate coated by a ferroelectric film and which network has a complementary transmission line consisting of three metal strips separated by gaps. It is stated that the various combinations of the amplitude and phase modulation can be obtained to compensate a parasitic amplitude modulation, and also the microwave amplitude may be effectively controlled. However, the only feature given is that the phase-shifting device has one gap at least two times larger than the second gap in the complementary transmission line.
According to a third Russian document, SU 1030889, the phase-shifting network has its phase shifting range extended while its matching capability is improved up to several octaves, its physical size is small and its operational speed is improved, for use in phase modulation and commutation of measurement and communication technology. According to the title of the document it concerns UHF band radio communication, but not microwave applications. The main feature of this disclosure is that the device has recesses in mutually opposing metal strips forming periodic structures.
A document, U.S. Pat. No. 4,445,098, discloses a method and an apparatus for a fast switching dual-toroid microwave phase shifter. The document describes a phase-shifter comprising a linearizing PROM, a DAC and an amplifier connected to a ferrite phase-shifter. The PROM constitutes a programmable memory containing a unique linearizing function for the phase shifter. However a phase-shifter utilizing ferromagnetism will still be considerably slow and not be found very practical for phase modulation of fast bit-streams on to a carrier.
Finally a U.S. Pat. No. 5,210,775 discloses a xcfx80/4 shift QPSK-modulator and communication apparatus for receiving digital signals and outputting a modulated signal in accordance with each of the two inputted digital signals representing a quadrature signal.
Therefore there is still a demand for a device being able to modulate a microwave signal from a fast digital bit-stream of the order hundreds of MHz and above, without the need for a conventional frequency up-conversion, in order to obtain a better reliability and stability than a conventional solution will offer, even at a lower cost.
Thus, the object of the present invention is to make possible directly, onto a microwave carrier right at the operating frequency, to modulate an information signal from a digital bit-stream without the use of a conventional up-conversion chain or frequency multiplication after the modulation. According to the invention a fast ferroelectric phase alternator is provided and operated with a variable voltage source controlled directly by the digital bit-stream representing the information to be modulated.
The method of the present invention is set forth by the independent claims 1 and 4, and further steps are set forth by the dependent claims 2-3 and 5-8.
An arrangement for modulating a microwave carrier by a fast digital bit-stream according to the present invention is set forth by the independent claims 9 and 12 while further embodiments of the arrangement are embraced by the dependent claims 10-11 and to 13-16.